74 Dart Seatbelts, how do they work correctly?

'74 was the first year for the "unibelt" combined lap and shoulder belt. The most charitable thing to be said about them is that they're not as godawful as the '72-'73 separate clip-together retractable lap and nonretractable shoulder belts, which in turn were somewhat less godawful than the completely separate '69-'71 lap and shoulder belts. See this post to try and get your '74 belts back in working condition. Keep in mind that they were very finicky even when brand new; if you pulled the belt out too fast or too suddenly, they'd lock and force you to return them all the way to the retractor and start again. Damnuisance.

I used to recommend retrofitting 4-point Wesco belts as I did on my '71, but not any more (the article is correct in its facts and physics, and everything said about 5-point harnesses is equally true of the 4-point variety, except the additional injuries that a 5-point can do to your 'nads).

When I get rid of the separate lap/shoulder straps in my '73, I'm putting in these up-to-date, reputable-brand ECE-approved 3-point seat belts (linked vendor is reliable; I've been buying there for a decade; their eBay store is here). More info on belt selection and mounting is at the maker's website. If I needed mounting hardware, I'd buy it again from Wesco (and their page has good illustrated write-ups on how to install belts in pillarless cars like yours here ), but the Securon belts are a much better-designed product than anything Wesco carries.

There are also bench- and bucket-seat 3-point belt setups available from XV Motorsports, but I am hesitant to recommend or use them until I have more than a one-line say-so from the company that their belts actually meet the relevant safety standards; discussion on that matter is here.